It is known that a hydraulic variable valve timing device varies a valve timing (opening/closing timing) of an intake valve and/or an exhaust valve for an output improvement of the internal combustion engine, fuel consumption reduction, and exhaust emission reduction. The following control is performed in a system including the variable valve timing device. When the target advanced amount of valve timing is less than a switching threshold, an amount of oil pressure control (control duty value) is controlled to a specified value so that a control mode is switched to a reference timing control mode in which an actual valve timing (actual advanced amount of valve timing) is hold at a reference timing. The switching threshold is established near the reference timing. When target advanced amount exceeds the switching threshold, the amount of oil pressure control is switched to the F/B control mode which performs feedback (“F/B”) control so that the difference of an actual valve timing and the target valve timing is decreased.
However, in the system which includes both an intake variable valve timing device and an exhaust variable valve timing device and supplies hydraulic pressure with a single hydraulic pump common to the oil pressure regulating valve of the intake side and the exhaust side, there are following problems. Namely, while one variable valve timing device is controlled by the F/B control mode, when the control mode of the other is switched, the hydraulic pressure (capacity) supplied to one variable valve timing device may be fluctuated. The actual valve timing may be fluctuated owing to this, and a drivability and an exhaust emission may be deteriorated.
In order to solve the above problems, JP-2005-98150A (U.S. Pat. No. 6,928,968B2) shows following system. When the target valve timing of at least one variable valve timing device is brought into the F/B control mode, the control modes of both variable valve timing devices are simultaneously switched to the F/B control mode. When the target valve timings of both variable valve timing devices are brought into the reference timing control mode, the control modes of the variable valve timing devices are simultaneously switched to the reference timing control mode. Thereby, while one variable valve timing device is controlled in the F/B control mode, it is prevented that the control mode of the other variable valve timing device is switched.
When the variable valve timing device is controlled in the reference timing control mode as shown in FIG. 2B, the amount of oil pressure control is controlled to the specified value so that the moving element of the variable valve timing device is pressed against the stopper part. Thereby, since actual valve timing is held in the reference timing, for example, most retarded timing, the circulation amount of the oil (oil leakage) increases and the amount of oil storage in the oil pan decreases.
On the other hand, when the variable valve timing device is controlled in the F/B control mode as shown in FIG. 2A, the F/B control of oil pressure control is performed so that the difference between the actual valve timing and the target valve timing may be decreased. Thereby, since actual valve timing is controlled at a vicinity of the target valve timing, the oil circulation amount (oil supply to the variable valve timing device) decreases, and the of oil storage amount in the oil pan increases. Hence, most of the crankshaft may be immersed in the oil stored in the oil pan, the mechanical rotational resistance of the crankshaft may increase, the torque loss of the internal combustion engine may increase, and the output may be deteriorated.
However, in the system shown in JP-2005-98150A, there is a problem that the period in which the variable valve timing device is controlled in the F/B control mode is prolonged, so that the power of the engine is deteriorated due to the increase in the oil storage amount.